Known methods for calibrating a so-called rotary encoder, which is an angle detector having a scale reader head on the periphery of a rotating divided circle, include methods that use comparison calibration, the equally divided average method, and other methods. Calibration using the equally divided average method is described in Patent Documents 1 and 2.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2006-098392
[Patent Document 2] Japanese Laid-open Patent Publication No. 2011-099804
A sequential two-point method may be used to calibrate a rotary encoder. When the sequential two-point method is used, calibration can be performed using two scale reader heads. Calibration (calculation of scale error) of a rotary encoder by the sequential two-point method requires fewer scale reader heads than other calibration methods. The calculations involved in calibration are also simpler in the sequential two-point method than in other calibration methods.
The sequential two-point method has not been used conventionally as a method for calibrating rotary encoders. The main reason for this is that the error component in the integer interval of equal division of the angle interval between the two scale reader heads is lost from the calibration result.
Specifically, the scale error of a rotary encoder is composed of harmonic components having a single rotation as the fundamental period. The angle interval between two scale reader heads (the angle interval between the scale reading positions of the two scale reader heads) may be the angle interval obtained by dividing one rotation, which is the full range of measurement, into k equal portions, where k is a positive integer. In this case, the sampling theorem imposes a limitation in that only frequency components up to the k/2th frequency component can be handled as the error component of the scale error. There is also the limitation that it is impossible to detect frequency components in the error component of the scale error that are on the order of integer multiples of k. These limitations are obstacles to the use of the sequential two-point method.
It appears that, in theory, this problem can be overcome by increasing the value of k. However, when the value of k is increased, sensitivity to the low-frequency component important to the scale error decreases, and since the accumulation of accidental error increases, the precision of calibration is reduced. Increasing the value of k is therefore not a good solution to the abovementioned problem.
An object of the present invention is to provide a self-calibration method for an angle detector, which makes it possible to precisely calculate the scale error at each angle position in the full range of measurement using a sequential two-point method.
An object of the present invention is to provide an angle detector that is provided with a self-calibration function, wherein self-calibration of the scale error is performed using a self-calibration method, whereby the scale error can be precisely calculated at each angle position in the full range of measurement using a sequential two-point method.
An object of the present invention is to provide a circumferential scale calibration device, whereby a circumferential scale is calibrated using a self-calibration method capable of precisely calculating the scale error at each angle position in the full range of measurement using a sequential two-point method.
An object of the present invention is to provide a calibration device for an angle detector, whereby the scale error of the angle detector is calibrated using a self-calibration method capable of precisely calculating the scale error at each angle position in the full range of measurement using a sequential two-point method.